1. Field of The Invention
The present invention relates to an improved non-single crystalline semiconductor material containing silicon atoms (Si) or/and germanium atoms (Ge) as a matrix and at least one kind of atoms selected from hydrogen atoms (H) and halogen atoms (X) and which has an average radius of 3.5 .ANG. or less as for mirovoids contained therein and has a microvoid density of 1.times.10.sup.19 (cm.sup.-3) or less. The non-single crystalline semiconductor material according to the present invention includes an amorphous silicon semiconductor material (which includes a microcrystalline silicon semiconductor material), a polycrystalline silicon semiconductor material, an amorphous germanium semiconductor material (which includes a microcrystalline germanium semiconductor material), a polycrystalline germanium semiconductor material, an amorphous silicon germanium semiconductor material (which includes a microcrystalline silicon germanium semiconductor material), and a polycrystalline silicon germanium semiconductor material, each containing at least either hydrogen atoms (H) or halogen atoms (X) and having an average radius of 3.5 .ANG. or less as for mirovoids contained therein and a microvoid density of 1.times.10.sup.19 (cm.sup.-3) or less.
Any of these non-single crystalline semiconductor materials according to the present invention excels in semiconductor characteristics and adhesion with other materials and is effectively usable as a constituent element of various semiconductor devices such as solar cells, photosensors, thin film transistors, electrophotographic light-receiving devices, and the like.
2. Related Background Art
Silicon-containing non-single crystalline semiconductor materials such as hydrogenated amorphous silicon materials (hereinafter referred to as "a-Si:H") and hydrogenated polycrystalline silicon materials (hereinafter referred to as "poly-Si:H"), germanium-containing non-single crystalline semiconductor materials such as hydrogenated amorphous germanium materials (hereinafter referred to as "a-Ge:H") and hydrogenated polycrystalline germanium materials (hereinafter referred to as "poly-Ge:H"), and other non-single crystalline semiconductor materials containing silicon and Germanium atoms such as hydrogenated amorphous silicon Germanium materials (hereinafter referred to as "a-SiGe:H") and hydrogenated polycrystalline silicon Germanium materials (hereinafter referred to as "poly-SiGe:H") have been used as photoelectric conversion elements or semiconductor elements of various electronic devices such as solar cells, thin film transistors, photosensors, electrophotographic light receiving devices, and the like.
Solar cells made of these non-single crystalline semiconductor materials have recently received much public attention because they supply clean energy without causing CO.sub.2 buildup as in the case of thermal power generation. However, none of these solar cells are satisfactory particularly in terms of resistance to light degradation and photoelectric conversion efficiency in order for them to be continuously usable as a daily power source. In view of this, there is an increased demand for providing a high quality Si- or/and Ge-containing non-single crystalline semiconductor material which is effectively usable as the semiconductor active layer of solar cells, which serves to absorb incident light and generates photocarriers causing a photoelectromotive force.
Incidentally, A. H. Mahan et al. have reported that as for the conventional solar cells and thin film transistors prepared using amorphous silicon semiconductor films, their constituent amorphous silicon semiconductor contains spherical microvoids of an average radius of 4 to 6 .ANG. and a microvoid density of more than 2.times.10.sup.19 (cm.sup.-3) (see, A. H. Mahan et al., IEEE TRANSACTIONS ON ELECTRON DEVICES, vol. 36, No. 12, December 1989, pp. 2859-2862; or A. H. Mahan et al., PHYSICAL REVIEW B, vol. 40, No. 17, December 1989-I, pp. 12024-12027). In these papers A. H. Mahan et al. mention that those microvoids are related to band-tail states or recombination centers (levels in the vicinity of the center of the band gap, in other words), that those microvoids contain hydrogen atoms (H) in a bonded states therein, and that such hydrogen atoms (H) in a bonded states are mobile within the microvoids, and this would be a factor of causing light degradation at the amorphous silicon film.
It should be noted that A. H. Mahan et al. have not discussed anything about germanium-containing non-single crystalline semiconductors.
The present inventors have examined microvoids contained in the conventional non-single crystalline silicon semiconductor and the conventional non-single crystalline germanium semiconductor using a commercially available scanning tunneling microscope (STM). As a result, it has been found that the microvoids contained in the non-single crystalline silicon semiconductor are either circular or elliptic in shape and of 1 to 3 atoms in depth and that the microvoids contained in the non-single crystalline germanium semiconductor are either circular or elliptic in shape and of 1 to 4 atoms in depth. And in view of the situation of atom arrangement at the peripheries of the microvoids, it has been found that a stress would be present at the peripheries of the microvoids in any of the two cases.
Further, the present inventors have examined the microvoids present in the conventional non-single crystalline germanium semiconductor by means of the small angle X-ray scattering method (SAXS). As a result, it has been found that the microvoids contained have an average radius exceeding 4 .ANG. and are of 2.times.10.sup.19 (cm.sup.-3) or more in density.
From these findings, it is considered that the conventional non-single crystalline silicon semiconductors and the conventional non-single crystalline germanium semiconductors are accompanied by distortions which are deviations from crystal bonds for a considerable number of the silicon atom bonds or the germanium atom bonds. And it is considered that such distortions result in causing the foregoing problems relating to resistance to light degradation and photoelectric conversion efficiency in the case of a solar cell prepared using the conventional non-single crystalline silicon semiconductors or the conventional non-single crystalline germanium semiconductors.
The present inventors have found the following facts through experiments. That is, either a non-single crystalline silicon semiconductor film or a non-single crystalline germanium semiconductor film accompanied by such distortions as above described is not satisfactory in doping efficiency because the distortions prevent a dopant of n-type or p-type from being activated upon doping the film with said dopant. In the case of doping said non-single crystalline silicon semiconductor film or non-single crystalline germanium semiconductor film with said dopant, the dopant is often trapped within the microvoids present in the film wherein the dopant is bonded without being activated. Such non-single crystalline silicon semiconductor film or non-single crystalline germanium semiconductor film which (1) is accompanied by the distortions or (2) contains such microvoids in which the dopant is trapped and bonded without being activated, is poor in charge mobility.